The pitch bearing, as a key component of the wind turbine, works together with the pitch drive device to change the blade angle (that is, change the pitch angle) to control the blade output power and ensure the safety of the blade. The pitch bearing needs to withstand alternating axial forces, radial forces, and large overturning moments in three directions. The load distribution is shown in Figure 1. The high-frequency vibration of the blades will also be directly transmitted to the pitch. bearings.
Figure 1 Schematic diagram of the load distribution of the pitch bearing
The working environment of the pitch bearing is very harsh (high temperature, high cold, high salt, high wind and sand, high humidity). To make it meet the requirement of 20 years of service life, the pitch bearing must be guaranteed Maintain uniform and stable starting friction torque, effective sealing, and good lubrication throughout the entire operation period. Since megawatt wind power pitch bearings require high reliability and long service life and are double-row four-point contact ball structures, the technical requirements for finished products and parts are different from ordinary slewing bearings .
1. Characteristics of the pitch bearing
The pitch bearing connects the rotor hub and the rotor blades. Its main purpose is to achieve indexing or positioning of the turbine blades to optimize the blade angle at different wind speeds. The biggest feature of the pitch bearing is the peach-shaped raceway and the large number of rolling elements. The single row four-point contact or double row eight-point contact design of this type of bearing provides excellent load-bearing capacity. The steel ball has multiple contact points with the raceway, which allows the bearing to bear radial, axial and Overturning load. The main characteristics of the pitch bearing are:
• The blades are connected to the hub ( fairing ) and adjust the blade windward angle;
• Rotate through internal/external spur gears or hydraulic plungers;
• At a very small angle (<>
• Stationary for a long time and constantly vibrated;
• Bearing maintenance is difficult, and it must be maintained regularly every 6 to 12 months (direct observation);
• Exposed to In all weather conditions;
• The hollow cast iron hub and composite blades are very flexible and provide little bearing support;
• Designed for a 20-year service life (approximately 175,000 hours).
2. Causes of pitch bearing failure
In recent years, pitch bearing failures have been on the rise. This is partly related to the lack of knowledge in the early pitch bearing industry. For example, in the early days, only engineering algorithms were used to check the strength of the pitch bearing raceway. , finite element analysis , etc. are not introduced. The classic failure mode (fatigue spalling) predicted by standard bearing calculation models is actually a very rare cause of failure. Studies have shown that pitch bearing failure is often related to lubricant degradation, lack of structural flexibility, improper loading and operating application.
2.1 Lubrication Failure
The classic failure modes predicted by standard bearing calculation models (i.e. fatigue spalling, indentation) are not actually common causes of pitch bearing failure. Common faults are usually caused by poor lubrication. Failures caused by lubrication include fretting (pseudo Brinell indentation), corrosion, pitting and surface-induced fatigue.
pitting
false indentations and corrosion
dents and Corrosion
Corrosion pitting
Surface induced fatigue
2.2 Overload operation
Because the bearing lacks the rigid support of the hub assembly, resulting in imbalance, part of the raceway bears most of the load. Failures caused by improper loading and operation include damaged parts (rolling elements, cages, raceways), cage locking and raceway damage. Of course, poor lubrication conditions will also exacerbate these failures.
Raceway contact fracture
Raceway contact part damage, resulting in cold processing
raceway broken
cage broken
steel ball broken
2.3 Elliptical truncation failure
In the pitch bearing, the contact area between the steel ball and the raceway forms an ellipse, which is centered on the raceway contact angle. Under larger thrust or overturning loads, the contact ellipse can exceed the physical limits of the raceway (cutoff). The potential for contact truncation increases with the ratio of bearing diameter to thickness, or with a reduction in external support. Severe contact truncation can produce stress rises that can lead to raceway edge fractures or ball fragmentation.
Oval (undamaged)
Oval (before failure)
2.4 Fatigue damage
Bearing subsurface cracks occur under the action of alternating shear stress. The cracks expand outward under load, eventually causing the contact surface to peel off.
2.5 Plastic deformation
Axial load, radial load and overturning moment are not well distributed on the pitch bearing, resulting in plastic deformation.
2.6 Raceway wear
Impurities, dust, abrasive that cannot be filtered, and vibration of the blades cause pitting and pits in the pitch bearing.
2.7 cage fracture
Due to cage material and manufacturing problems, the pitch bearing produces relative deformation of the inner and outer rings under load, and the cage quickly fails after being subjected to the tension generated by the relative deformation of the inner and outer rings.
2.8 Ring fracture
When the pitch bearing has design or manufacturing defects or is overloaded, the bearing ring will break under load.
3. Technical methods to prevent pitch bearing failure
Most pitch bearings fail in a similar manner, but the underlying causes may vary, so it is important to start by understanding the unique problem of the bearing. Depending on the specific issues with a given bearing, bearing upgrades should include some or all of the following technical improvements.
3.1 Raceway contact area
Increasing the raceway contact area can minimize or eliminate contact truncation. In addition, enhancements in bearing materials can also reduce deformation.
Comparison before and after the bearing upgrade, contact truncation is indicated in red
3.2 Cage design
Integral cage Although there are some theoretical advantages, taking into account manufacturing accuracy, material strength , cost and other factors, segmented retaining The frame has greater advantages in reality. The segmented cage allows limited movement space, which can reduce tensile and compressive loads; high-strength alloy steel can be used to improve durability and reduce contact wear.
3.3 Optimization of raceway geometric parameters
improves load distribution and balance by controlling raceway geometric dimensions and tolerances. Optimized raceway parameters can reduce slip and friction, thereby reducing internal wear and improving the response and efficiency of the pitch system.
3.4 High durability seal design and raceway hardening
adopts "H" shaped cross-sectional profile and labyrinth seal ring to replace the original ordinary sealing structure (picture below); the floating design is highly responsive and can provide sealing even if deformed. sealing pressure; in terms of materials, the sealing ring uses wear-resistant, thermoplastic polyurethane instead of the traditional nitrile rubber , which is more durable: preventing the entry of contaminants (grease, water, impurities), better sealing effect, higher strength, Able to extend service life and replacement intervals.
Improvement of sealing structure
3.4 Raceway hardening
Raceway surface hardening ( induction hardening ) can help prevent subsurface damage (yield) or internal damage, and the uniform hardness distribution of the raceway surface can withstand heavy loads.
3.5 Packaging
Proper packaging can prevent corrosion and damage caused by shock, vibration and other hazards during transportation. Packaging should include applying anti-corrosion coating on the mounting holes; wrapping the bearings with VCI anti-rust paper; packaging in vacuum sealed bags; using individual crates (stacked in two layers).
Summary: The optimization idea is as shown in the figure
Bearings for wind turbines are as shown in the figure
